An article appeared on page 510, volume 11, of the Philosophical Magazine in 1881 in which Alexander Graham Bell reported upon certain experiments which he conducted to study the sound-emitting properties of materials when exposed to the action of rapidly interrupted sunlight. Bell, among other things, introduced samples of material into a chamber and passed an intermittent beam of sunlight therein to produce audible effects. Bell observed that when the sample was in an acoustical resonant chamber the maximum response was achieved at a frequency of interruption equivalent to the resonant frequency of the chamber. While Bell was primarily interested in producing sound, he did recognize that this effect could be employed to study the properties of the material.
In my copending patent application, referred to above, I disclosed a method and apparatus for analyzing substances which employed the photo-acoustic phenomenon first recognized by Bell but apparently not developed by others thereafter.
In the devices discussed by Bell, and in my previous devices, the sample to be analyzed (or which was to respond to the light) was included in a chamber which was made sufficiently long to be resonant at the frequency of interest. Upon further study of the photo-acoustic phenomenon, I have discovered that substantially greater signals can be obtained by limiting the volume of fluid in the cell and that limitation of such volume can most optimally be obtained by designing the sample holding chamber and the remainder of the device forming the resonant structure to optimize the overall cell volume.
I have also found that further improvement of the signal response can be achieved as shall be discussed below.
I have also realized that the new cell structure which results from the above-mentioned discoveries enable the photo-acoustic effect to be employed in chromatography as well as direct analysis of samples such as skin with an open-ended cell.
Therefore, it is an object of this invention to provide improved photo-acoustic cells.
It is a further object of this invention to produce improved photo-acoustic cells with increased signal response.
It is still a further object of this invention to provide improved photo-acoustic cells which can be applied to a wide variety to uses.